Charlie Gordon originally started off with an IQ of 68 which in current thiinking is mentally retarded. The typical average IQ is between 80 and 120. Anything below or above those numbers is atypical. An IQ above 130 is considered to be gifted, and an IQ below 70 is considered to be Mentally retarded.
Charlie has a surgery which is supposed to make him smarter which it does for a while. After a time, the progress made by the surgery wears off, and Charlie is once again mentally retarded.
The answer to the "why" part of your question is that the surgery only worked for a short time.